The present invention relates to an apparatus for quantitatively measuring characteristic values of a medium by sending an ultrasonic wave pulse into a medium such as a human body, etc., receiving the pulse wave sequentially reflected from respective depths and by processing such received signal, and particularly, to a signal processing system where spectrum error resulting from the fact that a degree of convergence (divergence) of the ultrasonic beam changes non-uniformly as a function of the depth of the medium can be corrected on realtime basis in the time domain without obtaining the shape of the spectrum by FFT, etc.
First, changes in the convergence of the ultrasonic beam dependent on the depth of the medium will be explained. In FIG. 1(a), 1 denotes an ultrasonic wave transducer consisting of a piezoelectric element such as PZT, etc. and 2 is a medium such as a human body. The primary ultrasonic pulse sent from the transducer 1 travels in the depth direction (z-axis direction in the figure) of the medium, secondarily generating waves reflected from the medium at respective depths and finally disappears. The reflected waves from respective depths travel in a direction opposite to the primary pulse and are received by the transducer 1. A wave reflected from the shallow area with a small value of z reaches the transducer quickly but a wave reflected from the deep area with a large value of z reaches with a delay. Therefore, the received signal has a continuous waveform. The characteristic values of a medium such an ultrasonic wave attenuation coefficient, etc., of living body tissue can be obtained by processing the received signal. It is obvious that the attenuation coefficient is proportional to frequency in living body tissue, etc., and this proportional coefficient is called a slope of attenuation coefficient and therefore, it is often used as a characteristic value of the medium.
As explained above, when the primary ultrasonic pulses used are reflected from a certain depth in the same medium, the shape of the spectrum and intensity of the wave is always assumed to be constant irrespective of the depth in order to check the frequency characteristic of the medium, that is, there is no attenuation in transmitting function of medium and it does not depend on frequency.
In actual practice, however, the shape of ultrasonic beam formed depending on the sending and receiving sensitivity of ultrasonic pulse is not constant as a function of depth with respect to some frequency components and changes, as shown in FIG. 1(a). Moreover, a sound pressure along the center axis is distributed, as shown in FIG. 1(b). The beam also includes, as indicated by a dotted line shown in FIG. 1(a), other frequency components and therefore, a sound pressure on the center axis is distributed with an actual difference from that of FIG. 1(b). This is because the degree of convergence (namely, degree of divergence) of beam changes geometrically in three dimensions If this effect is neglected, a large error is introduced into the distribution in the depth direction of the measured characteristic values of the medium.
To correcting such an error, the inventors have proposed the following system (*1: Miwa et al., Japanese laid-Open Pat. No. 58-55850, corresponding to U.S. Pat. No. 4,509,524).
In this system, a reference medium like water, which has attenuation so small that it can be neglected (it is desirable that the acoustic impedance and sound velocity are as similar as possible to the medium to be measured but if there is a difference, it can be allowed for) is used. The ultrasonic pulse is transmitted from the transducer, the reference reflector (a solid flat plate or ball having smooth or rough surface) is placed at various depths z (where distance between the transducer and reflector on the beam line of the transducer is z), and the respective reflected waves are received and spectrums Sz(f) are also obtained where f represents frequency A reference depth, z.sub.0, for example, near the focus shown in FIG. 1, is designated and the spectrum of reflected wave at the depth z.sub.0 is Sz.sub.0 (f). In the spectrum domain, Sz(f).sup.2 is standardized as Sz.sub.0 (f).sup.2. A value Gz(f) obtained from the standardization is called a geometrical factor (G factor) and is defined as indicated below. EQU Gz(f)=.vertline.Sz(f).vertline..sup.2 /.vertline.Sz.sub.0 (f).vertline..sup.2 ( 1)
.vertline.S'z(f).vertline..sup.2 is defined as the power spectrum as defined by the equation (2). EQU .vertline.S'z(f).vertline..sup.2 =.vertline.Sz(f).vertline..sup.2 /Gz(f) (2)
In this definition, S'z(f) has the same spectrum shape and intensity, namely Sz.sub.0 (f), at all depths. An error due to the geometrical factor can be corrected by measuring the processing characteristic values of the medium through investigation Concerning how S'z(f) changes in accordance with the medium being measured.
The concept for correction with respect to media is introduced into a successive application (*2: Miwa, Japanese Patent Application No. 57-57573) and the relationship indicated in equation (3) below, is used in place of the equation (1) and on the power function. ##EQU1##
This concept was applied in two other patent applications (*3: Miwa, Ueda, Japanese Patent Application No 57-129902, Miwa et al., corresponding to U.S. patent application Ser. No. 477,935, filed in 1983) and was an aspect of diffraction correction in the following three reference papers given at the Eighth International Symposium on Ultrasonic Imaging and Tissue Characterization, June 5-8, 1983; Ultrasonic Imaging 5, P. 186-187 in June, 1983.
*5 (D. W. Pettibone et al., Diffraction effects on the measured spectrum of a focused acoustic transducer) PA1 *6 (Cloosterman, J. M. Thijsen et al., IN VITRO absolute attenuation measurement with diffraction correction) PA1 *7 (M. Fink et al., Influence of diffraction effects on the estimation of the tissue attenuation by spectral analysis of A-lines)
However, the references *3, *4, *5, *6 and *7 all describe signal processing in the frequency domain and correction of diffraction effects in the frequency domain. The reference *7 reports on a simplified approximating method where the center frequency is obtained accurately from the center frequency of the spectrum divided by Gz(f) to correct the center frequency obtained from the power spectrum. In this method, water is used as no-attenuation medium and the deviation of the center frequency of the spectrum Sz(f) of the signals reflected from respective depths from the center frequency of spectrum Sz.sub.0 (f) of the reference depth z.sub.0 is obtained and the deviation is added to the center frequency of the received signal at the respective depths obtained from the medium to be measured to perform the correction. This method is equivalent to an approximate multiplication in which any errors inevitably increase in magnitude.
In the reference *2, since the focus of the application is on power, the bandwidth of a wideband pulse is divided into n-domains (n&gt;3) or into n different frequency power components, consideration is taken for the geometrical factor in the power region but the shape of spectrum (in case n is sufficiently large) is not used or considered. When a sufficiently large n is used, conversion to the frequency domain by using an FFT, etc., is also necessary. This is an intermediate method between the frequency region method and the time region method described later.
When n is large in devices of the references *3, *7 and *2, the geometrical factor (G factor) or diffraction effect is corrected in the frequency domain during signal processing in the frequency region, and it requires that the waveform on the time axis be extracted using a time window (of window width T) around a certain point (t=2z/C corresponding to the depth z at the sound velocity C), the waveform is then converted into frequency domain data through a Fourier transform, and correction of G factor along with signal processing for extracting the attenuation coefficient slope are carried out. Therefore, a disadvantage arises in that a finite processing time is required even when an FFT (Fast Fourier Transform) circuit and other circuits are used, and, as a result, the received signal is not suited to real-time processing. Moreover, the FFT and other circuits are complicated, large in size and expensive.
The same inventors as in the present invention proposed a signal processing system where characteristic values of a medium are extracted from the received signal in the time region without converting it to the frequency region (*2, *8 and *9).
The reference *8 (Miwa et al., Japanese Laid-Open Patent No. 57-550) performs out signal processing in the time region on the received reflected power and obtains the attenuation slope of the medium. In this application, the reflectivity does not have a frequency characteristic of interest. In the frequency region, power is the 0th order moment of the power spectrum and is naturally influenced by the G factor. However, this factor is not considered in this application.
As is already explained, the reference *2 is capable of executing signal processing only in the time domain when n is 3. Although correction using the G factor is considered, the G factor takes the form of equation (3) instead of equation (1) because attention is only on the power as is the case in reference *8. As explained above, it is sufficient for the application field of the references *2 and *8 to use an amount correction only for integral values in the bandwidth as a whole, and the error due to so-called spectrum scalloping cannot be corrected in the references *2 and *8. The reference *9 can likely correct scalloping error in the time region system.
The reference *9 (Shiba et al., Japanese Patent Application No. 58-77226) discloses that the 0th, 1st, 2nd, 3rd, . . . moments of power spectrum are obtained from the in-phase component I and quadrature component Q of a quadrature detector output. This reference describes extracting characteristic values of the medium from these moments, using a simplified structure, reduced in size, cost and real-time processing.
It is necessary for correct medium characterization to correct for the G factor, even in reference *9, however, such a correction has not been reported yet in the art.